When the antidrug educator Tim Ryan talks to students, he often asks them what they know about marijuana. “It’s a plant,” is a common response.

But more recently, the answer has changed. Now they reply, “It’s legal in Colorado.”

These are confusing times for middle and high school students, who for most of their young lives have been lectured about the perils of substance abuse, particularly marijuana. Now it seems that the adults in their lives have done an about-face.

Recreational marijuana is legal in Colorado and in Washington, and many other states have approved it for medical use. Lawmakers, the news media and even parents are debating the merits of full-scale legalization.

“They are growing up in a generation where marijuana used to be bad, and maybe now it’s not bad,” said Mr. Ryan, a senior prevention specialist with FCD Educational Services, an antidrug group that works with students in the classroom.

“Their parents are telling them not to do it, but they may be supporting legalization of it at the same time.”

Antidrug advocates say efforts to legalize marijuana have created new challenges as they work to educate teenagers and their parents about the unique risks that alcohol, marijuana and other drugs pose to the developing teenage brain.

These educators say their goal is not to vilify marijuana or take a stand on legalization; instead, they say their role is to convince young people and their parents that the use of drugs is not just a moral or legal issue, but a significant health issue.

“The health risks are real,” said Steve Pasierb, the chief executive of the Partnership for Drug-Free Kids. “Every passing year, science unearths more health risks about why any form of substance use is unhealthy for young people.”

Already nearly half of teenagers — 44 percent — have tried marijuana at least once, according to data from the partnership. Regular use is less common. One in four teenagers report using marijuana in the past month, and 7 percent report frequent use — at least 20 times in the past month.

Even in the states where marijuana is legal, it remains, like alcohol, off-limits to anyone younger than 21. But the reality is that once a product becomes legal, it becomes much easier for underage users to obtain it.

This summer, the Partnership for Drug-Free Kids released its annual tracking study, in which young people were asked what stopped them from trying drugs. Getting into trouble with the law and disappointing their parents were cited as the two most common reason young people did not use marijuana. The concern now is that legalization will remove an important mental barrier that keeps adolescents from trying marijuana at a young age.

“Making it legal makes it much more accessible, more available,” said Dr. Nora Volkow, the director of the National Institute on Drug Abuse. “This is the reality, so what we need to do is to prevent the damage or at least minimize it as much as possible.”

Drug prevention experts say the “Just Say No” approach of the 1980s does not work. The goal of parents should not be to prevent their kids from ever trying marijuana.

Instead, the focus should be on practical reasons to delay use of any mind-altering substance, including alcohol, until they are older.

The reason is that young brains continue to develop until the early 20s, and young people who start using alcohol or marijuana in their teens are far more vulnerable to long-term substance-abuse problems.

The brain is still wiring itself during adolescence, and marijuana — or any drug use — during this period essentially trains the reward system to embrace a mind-altering chemical.

“We know that 90 percent of adults who are addicted began use in teenage years,” Mr. Pasierb said. “They programmed the reward and drive center of their teenage brain that this is one of those things that rewards and drives me like food does, like sex does.”

Studies in New Zealand and Canada have found that marijuana use in the teenage years can result in lost I.Q. points. Mr. Pasierb says the current generation of young people are high achievers and are interested in the scientific evidence about how substance use can affect intelligence.

“You have to focus on brain maturation,” he said. “This generation of kids wants good brains; they want to get into better schools. Talk to a junior or senior about whether marijuana use shaves a couple points off their SATs, and they will listen to you.”

Because early exposure to marijuana can change the trajectory of brain development, even a few years of delaying use in the teen years is better. Research shows that young adults who smoked pot regularly before the age of 16 performed significantly worse on cognitive function tests than those who started smoking in their later teenage years.

Drug educators say that one benefit of the legalization talk is that it may lead to more research on the health effects of marijuana on young people and more funding for antidrug campaigns.

The Partnership for Drug-Free Kids plans to continue its “Above the Influence” marketing campaign, which studies show has been an effective way of reaching teenagers about the risks of drug use. The campaign does not target a specific drug, but it teaches parents and teens about the health effects of early drug use and tries to empower teens to make good choices.

“Legalization is going to make the work we do even more relevant,” Mr. Pasierb said. “It’s part of the changing drug landscape.”